Preferential sulphatization of complex ores



Patented Mar. 31, 1936 UNITED STATES PATENT OFF-ICE PREFERENTIALSULPHATIZATION OF COMPLEX ORES Sylvester James Broderick and Earl H.Brown, Yellow Springs, Ohio, assignors to Bethlehem Mines Corporation, acorporation of Delaware No Drawing. Application April 19, 1933, SerialNo. 666,846

Claims. (Cl. 75116) This invention relates to the preferential orwithout incurring serious losses of soluble iron selectivesulphatization of mixed metallic oxides in the leach solution.

and silicates. More specifically the invention re- Previousinvestigators, having in view a seleclates to the treatment of complexores, which betive sulphatization of non-ferrous oxides and cause oftheir earthy character and the intimate silicates, associated with ironoxides, have used association of their mineral constituents, are not asindirect sulphatizing agents, metallic sulamenable to known methods ofore dressing and phides and sulphates, alkaline metal sulphates,separation. It is particularly directed to the or their chemicalequivalents; as more direct hydrated type of ore, consisting of acomplex sulphatizers, they have used mixtures of sul- 10 aggregate ofminerals, including metallic oxides phur dioxide and air. All of thesehave been 10 and hydro-silicates, in the smelting of which it utilizedunder various conditions within limits is impossible to avoid reductionof the minor of temperature, time and concentration. Howmetalliccomponents, the latter therefore apever, the fact that the partialpressure of sulpearing in the product as alloying impurities. phurtrioxide (which is the effective reagent ]5 A an example f th typ of oreto which from whatever source derived) is an important the invention isparticularly directed, may be factor in any system of selectivesulphatization cited the nickel chromiferous iron ores of Cuba, and thatit must be effectively Controlled, Seems Greece and Borneo. These oresrange in analysis, to have been entirely ignored. This fact has on drybasis; iron 46 to 52, chromium 2 to 3, now been fully demonstrated inthe course of ni k l and cobalt up to 1.2%. These deposits experimentsin the sulphatizing of ores, and the 20 represent large tonnages ofcomparatively high knowledge thereby g haS been u y pgrade iron ore,which it has hitherto been found p ied to an adeq Solution of the p Inimpossible to utilize to any substantial degree in the case f the Cubano for example. it has the manufacture of commercial steel products, beenShown that nickel and alumina y be Such use having been limited toSpecial products moved and the ore beneficiated to a degree adein whichthe content of nickel and chromium quate for normal steel makingOperations by could be fiecti ely utilized exercising the proper controlof partial pressure A purpose of the invention is therefore to re- 21temperature during the sulpha'fizmg reacions.

52233153? nztttizrzzztarrr i a t of 30 course of Stee1 making operationscomponents, wh1ch it is desired to sulphat ze in Certain Ores of thischaracter, tend to run a preferential or selective manner, complicateshigher in alumina, than is desirable for normal h problem of mmmmng thedegree of 9 smelting practice; a further purpose of this inslon of therefpective comppnerits to.thelr solu' vention is therefore to effectsome decrease in me sulphates partlculafly 1S thls so m the case thealumina of the readily sulphatizable iron, which is pres- The processalso admits of chromium removal em in predominant proportlons Thedlmculty of control can be appreciated, if due considerabut F f degree@mmensura't'e wlth 1055 Iron tion be given to the fact, that conversionto sul- 40 AS speclfic purpose. of the i i to phate is reversible withincertain ranges of tem- 40 mamtam as nearly as posslble. the Orlgmal Ironperature, to the extent that the sulphate formed, i of the the processcohtemplaies only tends to dissociate. Such dissociation, dependanmcldental removal of chromlum which i ing on the particular element fromwhich the merit m however be removed to an effective sulphate isderived, may take the form of a direct flegree subseqllent processsteps, Well known reversion to the original oxide or, in certain cases,45 A it l g -t t t H d to the reduced metal; 01', again, it may passnown me 0 0 Tea mg me a 10 0X1 es throu h an intermediate sta e as basicsul hate, and silicates, so as to facilitate their removal, is beforereverting to the oxidi Whatever S the to nv r them to soluble ulpha byin mechanism of dissociation, it 'is necessarily actlgem alt 1Tsuitabletemperaltuigs 1n thef prese l ce companied by a secondary dissociationof the S03 50 0 a $11 D a wing agen I1 8 Case 0 C p 6X radicle set freeby the reversion of the sulphate, iron ores, such as those abovereferred to, the so th t, ulphur dioxide and Oxygen are propurpose ofsulphatization is to effect a preferduced. ential conversion of certainundesired compo- It is well known that the dissociation pressures nentsto soluble sulphates, removable by leaching, of various sulphates differaccording to the metal 5 pure oxides.

from which they are derived. From previous investigations of pairedsulphates, heated to various temperatures, it is known that there arealso variations in the pressures set up by different types of mixedsulphates. With certain exceptions, the dissociation pressure isgenerally that due to the less stable member of the pair. On the otherhand pairs of certain isomorphous sulphates exhibit the characteristicsof a single phase, having a well defined dissociation pressure,intermediate in range between that of the individual members. From thisit becomes evident, that, in the case of mixed non-isomorphoussulphates, the member having the highest dissociation pressure, that isto say, the least stable at the selected temperature, is the morereadily decomposed to its insoluble form; the other sulphate thereforeremaining substantially unaffected.

To better understand the distinctive features of the process hereindisclosed, let it be assumed that the ore exists in a simplified formand that its important components, iron oxide, alumina, chromic oxideand nickel oxide are present as By selecting the proper temperature andpartial S03 pressure, it should be possible to sulphatize one or more ofthe components in a preferential manner. For example, at a temperatureof 700 C. and S03 partial pressure of 120 m/m, only the oxides of nickeland aluminum should be sulphatizable. Conversely, the reaction beingreversible, it can be shown that under conditions of completesulphatization of the mixture, the comparative stabilities againstreversion, of the sulphates so formed, would run in the order: chromium,iron (ferric), aluminum and nickel, with nickel sulphate as the moststable.

In actual fact however, a complex ore contains other components, silicafor example, which intervene and change the characteristic dissociationpressures of the metallic sulphates above referred to. Also thereactivity to sulphatization of each component is appreciably affectedby the particular mineralogical form in which it exists in the ore.Nevertheless the general indications above cited hold true, as to thefeasibility of selective suphatization and a controlled reversion of thesulphates formed, so long as the components to be separated do not havethe character of solid solutions. While the investigations of single andmixed sulphates, above referred to, relate to the total dissociationpressures set up, it is evident that for the purpose in view, viz. acontrol of the formation of sulphates and their maintenance in a stableform, it is the partial pressure of sulphur trioxide that is the morereadily controlled factor.

As a result, a preferred process has now been worked out for thetreatment of Cuban ores, such as those of the Mayari and Moa Baydistricts; the features of which are as follows.

As is well known, sulphur trioxide is produced by the combination ofsulphur dioxide and the oxygen of the air, in the presence of a suitablecatalyst, such as platinum or an oxide of vanadium, at a temperature ofabout 450 C., the purpose being to attain a high degree of conversion ofsulphur dioxide and the concentration of trioxide being dependent on theratio of air and dioxide used in the process. For the purpose of thisinvention, it has been found that in order to secure the desiredresults, the generation of the sulphur trioxide, to be used forsulphatizing the ore, should be effected at a point in the system,

entirely separate from the zone of sulphatization. The gas, so produced,is then brought into intimate contact with the ore to be sulphatized,said ore being heated to a predetermined range of temperature in afurnace of suitable type. A concentration of 25-30% S03, arising from a2:1 ratio of air and sulphur dioxide and a 90% conversion of S02, hasgiven particularly effective results in sulphatization; lowerconcentrations may be satisfactorily used, but the time factor insulphatization is correspondingly increased; for this reason it ispreferred not to lower the concentration below 15%.

An important feature of the present process is the somewhat higher rangeof sulphatizing temperatures, 650 C. to 760 C., than that usedheretofore. If reference be had to known methods of sulphatizing, byeffecting the conversion of sulphur dioxide in contact with the ore tobe sulphatized, or by mixing pyrite or similar sulphur bearing materialtherewith, it will be seen that the required degree of S02 conversioncannot be met. For example, in the case of burned pyrite, the percentageconversion of S02 to S03 at 450 C. is less than 20%; at 500-600 C. withthe same rate of flow, the conversion is still less than 50%; and at700-750 C. it drops to less than 20%. Whether pyrite be roasted with theore to be sulphatized, or whether a mixture of sulphur dioxide and airbe passed over the ore at 400-500" C., it is evident that the reactionrate is too slow for the purpose sought. Any extension of the treatmentperiod, to offset the low reaction rate tends to a correspondingincrease in the amount of iron sulphatized.

Furthermore the sulphatizing temperature is necessarily limited to thatat which the combination of S02 and 0 can best be effected, say 400 C.to 500 C. Moreover the conflict between the formation of sulphurtrioxide in contact wth the ore, the latter acting to some degreecatalytically, and the sulphatization of the ore by the gas so formed,makes it impossible to attain the desired control of S03 partialpressure. In other words, these known methods do not provide the bestconditions, neither in respect to S03 concentration, nor in respect totemperature, for effecting a selective conversion of nickel and otherminor components of the ore to a form suitable for their removal.

The sulphur trioxide gas is passed into a suitable roasting furnace, sodevised as to provide a maximum degree of contact between the oremixture and the sulphatizing gas. The ore, or advantageously a mixtureof ore of 5 to 15% of sodium chloride, should previously be brought tothe desired roasting temperature, 650-700 C. In heating the ore,moisture and water of hydration are driven off, their removal beingfacilitated by passing air over the mixture, during the early stages ofthe heating period.

The most effective rate of flow for the sulphatizing gas should bedetermined experimentally. In the initial stages of sulphatization S03is rapidly absorbed by the ore, but as sulphatization proceeds the rateof absorption slows down appreciably; hence the rate of gas-flow intothe furnace should be cut down accordingly. By observing the gas leavingthe furnace for indications of unconsumed S03, and by sampling the oreat different stages of the roast, these conditions may be adequatelycontrolled.

If sulphatization at 650-700 C. be prolonged until 90% or more of thenickel has been sulphatlzed, it will be found that a substantialproportion of the iron, varying from 20 to 50%, has also been convertedto soluble sulphate. To avoid high iron losses in the subsequentleaching of the roasted product, it is essential to cause the reversionof the ferric sulphate to an insoluble form of iron. This may beeffected by increasing the temperature during the later stages ofroasting to a degree sufficient to decompose, or dissociate, the ferricsulphate. Within certain limits of temperature, say up to 750 C., andprovided roasting at this higher temperature be not protracted for toolong a period, the greater part of the sulphatized iron may be caused torevert with but slight loss of sulphatized nickel.

A more precise and controllable method of preventing the decompositionof the nickel sulphate, one which is an important feature of thisinvention, consists in maintaining on the roasted prod-' uct during thedecomposition period, a degree of gas pressure in excess of thedissociation pres sure of nickel sulphate at that temperature, but lessthan the dissociation pressure of ferric sulphate.

In the case of Cuban ore, the roasting temperature in the final periodmay advantageously be increased about 50 0., say to a temperature of700-7 60 C., while maintaining a slow flow of S03 gas over thesulphatized ore, so as to impose thereon a total gas pressure, highenough to inhibit reversion of nickel sulphate. This imposed pressure,which may be controlled to the desired value by a valved outlet from thefurnace, is due to the combined partial pressures of S03 and of its owndecomposition products S02 and at that temperature, and also of theinert gas which accompanies the S03. It is known that the dissociationpressure of nickel sulphate at 750 C. is only about 38 m/m, which valueis the sum of partial pressures of S03, S02 and 0. On the other handferric sulphate has a dissociation pressure greatly exceeding the valuefor nickel sulphate even at substantially lower temperatures, whichvalue increases rapidly as it approaches 700 0., when it attainsapproximately 560 m/m. Chromium sulphate likewise has high dissociationpressures, whereas the individual pressures of the sulphates of cobalt,manganese and magnesium (which may also be present as oxides orsilicates in ores of this type) are still lower than nickel sulphate,within a temperature range of 700-760 C. Aluminum sulphate has apressure higher than nickel sulphate but substantially lower than thesulphates of iron and chromium.

From this it becomes evident that, the substantial difference betweenthe low dissociation pressures of nickel sulphate (also of cobalt,manganese and magnesium, when these are present in sulphatized form) andthe comparatively high pressures of iron and chromium sulphates, renderspossible the selective reversion of the latter group to their insolubleform, without greatly affecting the solubility of the nickel and othersoluble metallic salts of low dissociation pressure, by maintaining overthe mixture a total pressure intermediate between the extreme valuesaforesaid. In so far as soluble aluminum is concerned, the imposedpressure selected should have a value also higher than that, which wouldcorrespond to the dissociation of aluminum sulphate.

It will of course be understood that the values given above refer to.the individual dissociation pressures of the various sulphatizedcomponents. Because of their influence on each other and depending ontheir respective proportions, the dominant dissociation pressure of anyone group of mixed sulphates will vary from the values indicated. In thesame way, the presence of intervening components of the ore, silica forexample, will influence the values. However, the wide disparity betweenthe two groups, namely nickel, cobalt, manganese and magnesium on theone hand, and iron and chromium on the other, allows sufiicient latitudein which to select the gas pressure to be imposed. Such selection, inView of the complex factors involved, is best determined experimentallyand for practical purposes may be expressed as the total pressure,corresponding to a specific S03 partial pressure.

By Way of illustration it may be stated that when the decomposition ofnickel and iron sulphates was carried out at 750 C. the total pressurewas 737 m/m, corresponding to an S03 partial pressure of 237 m/m. Thepartial pressure of nickel sulphate at this temperature is but 4 m/m,that of ferric sulphate exceeds the total pressure figure of 737 m/m.Consequently the decomposition of ferric sulphate can continueunopposed, the nickel sulphate remaining practieally unaffected. Byactual analyses prior to, and subsequent to decomposition, it has beenshown that by maintaining the conditions above specified the incidentalreversion of nickel sulphate may be limited to 2.5% whereas the ironsulphate may be decomposed to the extent of 97-93%.

It is further to be observed that the above S03 partial pressure of 237m/m is close to, but slightly less than, that due to the decompositionof aluminum sulphate. From this, it is evident that decomposition of anyaluminum sulphate present would not be avoided, under the conditionsspecified. To maintain both nickel and aluminum in soluble condition assulphates, but revert substantially all the iron, it would be necessaryto maintain an S03 partial pressure in excess of that due to thedecomposition of aluminum sulphate at the decomposition temperatureselected. This can be effected by lowering the temperature ofdecomposition, say to 720 0., at which temperature the S03 partialpressure of aluminum sulphate would decrease to about 100 m/m; inconsequence any total pressure corresponding to an S03 partial pressurein excess of this figure, but lower than that due to the decompositionof ferric sulphate, would be suitable for inhibiting the decompositionof aluminum and nickel sulphates.

With the temperature range and degree of gas pressure established, thethird controllable factor, that of time, remains to be considered andparticularly in respect to the comparative durations of the two roastingperiods, the functions of which are broadly that of sulphatization andrestrained or selective decomposition. In the first period at the lowertemperature 650-700 C., the initial stages of nickel sulphatizationproceed rapidly but the reaction slows down as the absorption of S03becomes less. 0n the other hand prolongation of the time ofsulphatization beyond a certain time promotes a rapid increase in theamount of iron that may be sulphatized at that temperature. At thehigher temperature of 700-760 C. the sulphatization of nickel tends toaccelerate and at the same time reversion of sulphatized iron ispromoted. The most efiective combination of the two roasting periods issuch that, the first period results in oversulphatization of the ore tothe extent that sulphatization of the iron is permitted to proceed to asubstantial degree, before changing tothe highertemperature.

There is probably some additional sulphatization of nickel at the highertemperature which serves to insure a higher recovery of nickel in asoluble form, but the main purpose of the second period -is to decomposeiron sulphate without a concomitant decomposition of nickel sulphate.With the Cuban ore referred to, the best results have been obtained byroasting at the higher temperature with a slow and controlled flow ofS03 for a period representing from 40 to 50% of the total time ofroasting. In this manner over 90% of the nickel has been obtained in asoluble form, while the solubility of the iron has been limited to 8% orless. Aluminum to the extent of about '50% has also been renderedsoluble.

The principles of the process as outlined above, may be applied in analternative and equally effective manner, by utilizing the ferricsulphate formed in the first roasting period at 650-700 C., as thereagent in the second roasting period at 700760 C. This can beaccomplished by making an addition of raw ore to the product of thefirst period in amount, such that the nickel and aluminum in the addedore may be sulphated by the subsequent dissociation of the ferricsulphate, available in the initially oversulphated ore. The furnace isthen closed and heated to approximately 750 C. and a total gas pressuremaintained therein, which corresponds to the selected S03 partialpressure; gas in excess of this pressure being allowed to escape fromthe furnace through a suitable valved connection and recovered forfurther use on a subsequent batch of ore.

After sulphatization has been completed the treated mass is quicklycooled by quenching in water. This is necessary, because if the mass bepermitted to cool in the presence of S03 it may, in passing through thetemperature range at which iron sulphatizes readily, acquire ferricsulphate, a result it is particularly desired to avoid. On the otherhand if residual S03 be swept out by a current of air, there is dangerof decomposing some of the soluble nickel sulphate and a consequentlyless eflicient removal of nickel from the ore. The cool d mass issubmitted to a suitable water leaching for the removal of the solublesulphates or" nickel and aluminum. Recovery of these soluble salts maybe effected by any methods known in the art, by precipitation forexample, but such methods are outside the scope of the invention, themain purpose of which is to maintain sulphatization under propercontrol. Obviously the cooling and leaching may be combined as one step.The leached residue may be dried and agglomerated by nodulizing orsintering and is then in suitable condition for smelting in a blastfurnace.

In certain cases analysis of the leach solution has indicated that anappreciable amount of chromium sulphate may be brought into solution.However this necessarily involves some additional loss in iron, becauseof the known inactivity of chromic oxide, causing it to sulphatize at acomparatively slow rate, and also because the same measures that may betaken to inhibit sulphatization of iron or to promote its decompositionif sulphatized, will be similarly effective in respect to the chromium.Therefore, while admitting the feasibility for certain purposes, ofremoving chromium with some increase in soluble iron, by this process,the preferred practice of the invention is not directed to chromiumremoval in any substantial degree, said removal may be adequatelyeffected by known metallurgical processes.

It will be obvious to those skilled in the art that the process steps,herein described and the methods devised for controlling partialpressures in the reactions of sulphatization, are applicable to acomparatively wide range of metallic oxides and silicates specificallyand to mixtures of sulphatizable inorganic salts in general, selectivetreat ment of which is based, in part on a selective sulphatization tosoluble form, in part on a selective reversion therefrom to an insolublecondition.

The specific features of the invention are set forth in the followingclaims.

We claim:

1. A process for the selective sulphatization of metallic oxides andsilicates in iron ores containing them, which consists in, treating theore with sulphur trioxide gas at a temperature favoring the formation ofsoluble metal sulphates having comparatively low dissociation pressures,raising the temperature after a predetermined period, so as todissociate any incidental metal sulphates of high dissociation pressureformed at lower temperature, while maintaining thereon a predetermineddegree of reaction gas pressure to inhibit dissociation of those solublesulphates having low dissociation pressures, cooling the mass andremoving the undissociated soluble sulphates by leaching.

2. A process for the selective sulphatization of nickel oxides in ironores containing them, which consists in, treating the ore with sulphurtrioxide gas at a temperature favoring the formation of soluble nickelsulphate, raising the temperature after a predetermined period, so as todissociate any incidental iron sulphate formed at lower temperature,while maintaining thereon a predetermined degree of reaction gaspressure to inhibit dissociation of soluble nickel sulphate, cooling themass and removing the undissociated nickel sulphate by leaching.

3. A process for the selective sulphatization of nickel and aluminumoxides in chromiferous iron ores containing them, which consists in,treating the ore with sulphur trioxide gas at a temperature favoring theformation of soluble nickel and aluminum sulphates, raising thetemperature after a predetermined period, so as to dissociate anyincidental iron and chromium sulphates formed at lower temperatures,while maintaining thereon a predetermined degree of reaction gaspressure to inhibit dissociation of soluble nickel and aluminumsulphates, cooling the mass and removing the undissociated nickel andaluminum sulphates by leaching.

4. A process for the selective sulphatization of nickel and aluminumoxides in chromiferous iron ores containing them, which consists in,adding a small amount of an alkaline metal chloride to the ore, heatingthe mixture to a temperature of 650 C. to 700 C., passing thereoversulphur trioxide gas which has been formed prior to its contact with themixture, continuing said flow of gas and maintaining said temperatureuntil substantially all the nickel, a large proportion of the aluminumand minor proportions of the iron and the chromium have becomesulphatized to soluble metal salts, raising the temperature to from 700C. to 750 C. and continuing the flow of sulphur trioxide gas untilthesoluble iron and chromium of earlier sulphatization have been decomposedto insoluble forms, rapidly cooling the mixture and leaching the solublenickel and aluminum salts from the insoluble residue.

5. A process for the selective sulphatization of nickel and aluminumoxides in chromiferous iron ores containing them, which consists intreating the ore with sulphur trioxide gas at a temperature of 650 C. to700 C. for a predetermined period, until substantially all the nickel, alarge proportion of the aluminum, and minor proportions of the iron andchromium have been sulphatized, raising the temperature for a furtherperiod to a point not exceeding 760 C. to decompose iron and chromiumsulphates, While maintaining thereon a total gas pressure whichcorresponds to an S03 partial pressure greater than that of nickel andaluminum sulphates but less than that of iron and chromium sulphates,quenching the mass and removing the nickel and aluminum sulphates byleaching.

6. A process for the selective sulphatization of nickel and aluminumoxides in chromiferous iron ores containing them, which consists in,adding 5% to 15% of an alkaline metal chloride to the ore, treating themixture with sulphur trioxide gas at a temperature of 650 C. to 700 C.for a predetermined period, raising the temperature for a further periodto a point not exceeding 760 C. while maintaining thereon a total gaspressure which corresponds to an S03 partial pressure exceeding that ofnickel and aluminum sulphates but less than that of iron and chromiumsulphates, quenching the mass and removing the nickel and aluminumsulphates by leaching.

7. A process for the selective treatment of iron ores containing oxidesand silicates of nickel, whereby the nickel constituents may be renderedsoluble preferentially to iron, which consists in, treating the ore witha sulphatizing gas at a temperature to effect the sulphatization ofsubstantially all the nickel and from 20% to of the iron, then raisingthe temperature to decompose the sulphatized iron, while imposing on themass a total gas pressure exceeding that derivable from thedecomposition of sulphatized nickel at that temperature, controllingsaid total pressure by maintaining a flow of sulphatizing gas, thepartial pressure of which is that corresponding to the total pressuredesired.

8. A process for the selective sulphatization of nickel oxides andsilicates in iron ores containing them, which consists in, adding to theore a small amount of an alkaline metal chloride, heating the mixture toa temperature of 650 to 700 0., passing thereover a stream of sulphurtrioxide gas until substantially all the nickel and a minor proportionof the iron have been sulphatized, adding thereto fresh ore in amount,such that the sulphatizable components of the latter, when heated tosuitable temperature, may react with the gaseous decomposition productsof the sulphatized iron derived from the original ore, effecting saiddecomposition by increasing the temperature to a degree not exceeding760 C., while maintaining thereon such a degree of total pressure as tocorrespond to a sulphur trioxide partial pressure intermediate betweenthose of nickel sulphate and iron sulphate, and removing theundecornposed products of sulphatization by leaching.

9. In a sulphatizing roast for complex ores under conditions effecting asubstantial sulphatization of several components of the ore, the methodof selectively controlling the dissociation of the sulphatizedcomponents by maintaining thereon a degree of reaction gas pressure,greater than the dissociation pressures determined for such of thesulphatized components as it is desired to inhibit from dissociation,but inferior to the dissociation pressures of the sulphatized componentsto be dissociated.

10. A process for controlling the decomposition of mixed sulphates whichconsists in, heating the sulphates in an atmosphere of sulphur trioxideto a temperature in excess of their appropriate temperature ofsulphatization, inhibiting the decomposition of sulphates having lowerdissociation pressure by maintaining thereon a degree of gas pressure inexcess of said dissociation pressure but inferior to the higherdissociation pressure of those sulphates decomposition of which isdesired.

SYLVESTER JAMES BRODERICK. EARL H. BROWN.

